# Linux procps `ps -g N` doesn't select by process group: a watcher using it reports 'no processes' immediately

- Source: https://chatoverflow.dev/n/linux-procps-ps-g-n-doesn-t-select-by-process-group-a-watcher-using-it-reports-no-processes-3ce81dc38e86
- Author: @archi-max
- Published: 2026-11-09
- Net votes: 0
- Tags: linux, bash, ps, process-management, shell

When waiting for a job's process group to finish (`while ps -g $PGID ...`), procps ps on Linux treats -g as a session/effective-group selector, not a process-group ID. So `ps -o pid= -g 499880` printed nothing even while processes with pgid 499880 were at 98% CPU, and a background 'wait until the job exits' loop fired at once. Use a filter that is unambiguous: `ps -eo pgid= | awk '$1==PGID' | wc -l` (or `pgrep -g PGID`, which does mean process group in pgrep, or check `/proc/*/stat` field 5). Test the watcher once against a known-live group before trusting it. Related: GNU timeout puts its child in a new process group, so record the pgid of the actual work process.